1. Field of the Invention
The invention relates to a vehicle engine structure, and more specifically to a vehicle engine structure in which a driving axle for driving a driving wheel is passed through an oil pan mounted on the bottom of an engine.
2. Description of the Related Art
Vehicle engine structures of this kind are adopted in vehicles or the like in which front wheels are driving wheels, such as, in particular, four-wheel-drive vehicles. Japanese Patent Application Publication No. 2006-207400 (JP-A-2006-207400) and Japanese Patent Application Publication No. 11-101117 (JP-A-11-101117) describe vehicle engine structures in which a driving axle for driving a front wheel is horizontally passed through an oil pan mounted on the bottom of an engine.
With regard to four-wheel-drive vehicles, Japanese Patent Application Publication No. 11-229841 (JP-A-11-229841) and Japanese Patent Application Publication No. 8-100707 (JP-A-8-100707) describe vehicle engine structures in which a driving axle for driving a front wheel is longitudinally, that is, in the vehicle lengthwise direction, passed through an oil pan under a longitudinally-mounted engine. In such a vehicle, the power output from the engine through a transmission is distributed to the front wheel side and the rear wheel side by a transfer unit. The power is then transmitted to the rear wheels through a rear differential gear and driving axles, and transmitted to the front wheels through a front differential gear and driving axles.
A longitudinally-mounted engine that is used as an outboard motor or the like of machines other than vehicles, and that has a through hole formed through an oil pan in a substantially vertical direction, through which a drive shaft for driving a screw propeller is passed is also available.
FIG. 7 is a diagram showing an engine frame, an engine mount, and bottom rails of a V-type engine. In FIG. 7, E designates the engine frame, M1 and M2 designate the engine mount, and B1 and B2 designate the bottom rails. The right and rear directions in FIG. 7 are the right and rear directions with respect to the direction of travel of the vehicle.
With regard to the above-described vehicle engine structure, there has been a problem that, when the bottom rail portions are significantly, bendingly deformed near the longitudinal centre, at which the bottom rail portions of the engine body are supported by the engine mount, due to the force applied to a crankshaft bearing, such as when vibration including a vibration component of the engine frame vibration mode as shown in FIG. 7 is apt to occur because a V-type engine is used, the amplitude of vibration of the oil pan can become large, and the emitted sound of which the source is the oil pan can therefore becomes loud.
Specifically, when the engine operates, the force produced by combustion is transmitted to a piston, a connecting rod, and a crankshaft, vibrating the crankshaft bearing. For this reason, the oil pan, which is positioned near the crankshaft bearing and whose area is large, is vibrated in the lateral direction as shown in FIG. 7, thereby generating sound. In this way, the oil pan tends to become a noise emission source.
As a countermeasure for this, structural improvement can be carried out, such as rigidity enhancement near the point to which the force applied to the crankshaft bearing is transmitted. However, provision of an additional member, such as a separate ladder-like frame, leads to significant increase in mass of the engine and/or increase in cost for manufacturing the engine.
Alternatively, instead of enhancing the rigidity, a measure to reduce vibration by reducing the bearing clearance can be adopted. However, this measure can lead to worsening of fuel economy and/or cost increase due to increase in friction. In addition, because it is necessary to prevent seizure, reduction of the clearance is limited, and the effect of reducing vibration is therefore limited. Moreover, it becomes necessary to control the unevenness of quality, which results in increase in cost.